Myeloid-derived suppressor cells (MDSC) play a major role in immune defects associated with cancer. In cancer, normal myeloid cell differentiation is diverted from its intrinsic pathway of differentiation to mature myeloid cells (dendritic cells (DC), macrophages (M?), and granulocytes) towards differentiation of pathological MDSCs. In mice and humans, currently, two main MDSC populations have been characterized: monocytic MDSCs (M-MDSC) and polymorphonuclear MDSCs (PMN-MDSC). In na?ve mice, the same phenotype defines inflammatory monocytes (Mon) and neutrophils (PMN), respectively. In tumor-bearing (TB) mice, PMN-MDSCs are the prevalent population of MDSC. In cancer patients, MDSC have been isolated from the blood of patients with many different types of cancer. The frequency of MDSC correlates with a poor prognosis and progression in breast and colorectal cancer patients and, recently, was suggested as a biomarker of clinical response to cancer vaccine. Despite their morphologic similarity, PMN-MDSCs and PMNs, as well as M-MDSC and Mon, are functionally and phenotypically different; and MDSC, but not PMN or Mon, are immunosuppressive. Although many aspects of MDSC biology have been clarified in recent years, several fundamental unresolved issues remain. First, how to better distinguish MDSC in cancer patients and TB mice from PMN and Mon? Second, what drives the accumulation of MDSC in cancer? Third, what defines the survival of MDSC and how better to target these cells? Based on our recent preliminary data, we propose a novel concept of pathologic myeloid cell development in cancer. It suggests that, in contrast to physiologic conditions where bone marrow derived Mon differentiate in tissues to M? and DCs, M-MDSC in cancer preferentially differentiate to PMN-MDSC and represent one of the major sources of these cells. Our data suggest that this process is governed by epigenetic silencing of the retinoblastoma (Rb) gene. Our preliminary data demonstrated that PMN-MDSC in TB hosts have a much shorter lifespan than PMN. This effect was mediated by up-regulation of one of the TRAIL receptors (TRAIL-R) on these cells. We propose that pro- inflammatory factors drive up-regulation of TRAIL-R on MDSC. This suggests that TRAIL-R can be a specific target allowing elimination of MDSC, since it is known that normal PMN and other hematopoietic cells are resistant to agonistic TRAIL-R antibody or recombinant TRAIL. In this application we propose to test all these hypotheses in three specific aims. Specific aim 1. To investigate the role of Rb silencing in the regulation of myeloid cell differentiation in cancer; Specific aim 2. To study the mechanisms of regulation of MDSC survival in cancer mediated by TRAIL receptors; Specific aim 3. To study therapeutic targeting of MDSC by triggering TRAIL-receptors;